Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping Effects

Abstract

Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping Effects II-VI semiconductors have great importance in solar cell applications due to their excellent optical and electrical properties. This thesis is mainly concerned with the study of II-VI semiconductor thin films with a particular interest in their potential application in solar cells. A coating system based on close spaced sublimation (CSS) has been developed and thin films of zinc telluride (ZnTe), Zn and Te enriched ZnTe, cadmium sulfide (CdS) and cadmium zinc sulfide (CdZnS) were fabricated. CdS is transparent to electromagnetic radiations; in particular to the visible and infra-red regions and are highly resistive materials. This research work pertained to improve CdS thin films as window materials using close spaced sublimation technique. The optimization of deposition parameters including vacuum in the chamber, distance between source-substrate, source and substrate temperatures are all investigated. ZnTe has been used as a buffer layer between CdTe and the metal back contact in II-VI semiconductor solar cells due to its compatibility with p-type cadmium telluride (CdTe). The purpose of the buffer layer is to help CdTe form a good Ohmic contact with the metal back contact, however, ZnTe itself is highly resistive. The main goal is to reduce electrical resistivity of ZnTe for its use as buffer layer in the back contact. The resistivity of the ZnTe thin film is modified by doping with silver (Ag) and/or copper (Cu). The compositions of zinc (Zn) and tellurium (Te) in the enriched (Zn or Te) ZnTe thin film is also explored to lower the resistivity of ZnTe film, which has not been reported earlier using CSS technique. In all these processes, the structural, surface, electrical and optical properties are studied for strong correlation. Ion exchange process is adopted for Ag and Cu doping in as-deposited ZnTe thin films with subsequent annealing. CdS is a potential candidate for window layer due to its suitable and tunable energy band gap (2.42 eV). Effects of doping are investigated on the structural, electrical and optical properties of CdS thin films fabricated by the CSS technique. These properties of fabricated CdS thin films are found to be suitable for solar cell applications. To enhance band gap, CdS and Zn powder are mixed mechanically with different weight percentages to deposit thin films CZS fabricated by CSS technique that has not been documented earlier. The increased energy band gap for CZS is 2.57 eV, which has improved the window region.